Disposable pattern, composition for investment casting

ABSTRACT

A disposable pattern material for investment casting processes incorporating decachlorobiphenyl and a thermoplastic material; and an investment casting process utilizing a disposable pattern material incorporating decachlorobiphenyl.

United States Patent [1 1 Solomon June 3, 19 75 DISPOSABLE PATTERN, COMPOSITION FOR INVESTMENT CASTING [76] Inventor: Paul Solomon, 3500 University,

Highland Park, 111. 60035 [22] Filed: Oct. 13, 1972 21 1 Appl. N0.: 297,352

[52] US. Cl. 106/383; 106/386; 106/230;

106/270; 106/272; 260/28.5 A; 260/42 [51] Int. Cl. B28b 7/34; C08h 9/06 [58] Field of Search 106/3825, 38.8, 270, 272, 106/230; 26 0/28,5 A, 42

[56] References Cited UNITED STATES PATENTS 7/1964 l-lalpern et al 106/388 3,333,970 8/1967 Green 106/15 3,667,979 6/1972 Merges et al. 106/388 3,704,145 11/1972 Ware et al. 106/388 Primary Examiner-Lorenzo B. Hayes Attorney, Agent, or F irm-Dressler, Goldsmith, Clement & Gordon, Ltd.

57 ABSTRACT 4 Claims, No Drawings DISPOSABLE PATTERN, COMPOSITION FOR INVESTMENT CASTING This invention relates to improved pattern materials, to improved disposable patterns especially made for use in investment casting processes, sometimes also known as lost wax processes, and to an improved investment casting process.

Investment casting processes have been used for centuries. Materials for making disposable patterns to be used in such processes are formulated for a number of properties, including important properties such as dimensional reproduceability and highly accurate surface finish in the molded disposable pattern. Because such properties are critically important to many products made by lost wax processes, continuing efforts are always underway to improve those properties of pattern materials, among others.

Virtually all of the properties of an investment casting depend upon the quality of the disposable pattern. These in turn depend upon the characteristics of the pattern forming materials from which disposable patterns are molded.

Disposable thermoplastic patterns are usually formed by heating and melting a thermoplastic pattern forming composition, introducing the molten composition into a mold, and then cooling the composition until it solidifies to form a disposable pattern. Thereafter the disposable thermoplastic pattern is removed from the mold, is assembled if necessary with other patterns, and is then encased in a mold forming material, usually a ceramic material, in accordance with one of a variety of known methods, thereby to form a shell or cast about the disposable pattern. The disposable pattern is then removed, as by melting or vaporizing the pattern material, so that it leaves the shell or cast. Thereafter the shell or mold is ready for one-time use for forming an investment cast part. A text describing known procedures used in lost wax processes is entitled Investment Casting, H. T. Bidwell, Machinery Publishing Co., Ltd., England, 1969.

It is apparent that the surface characteristics of the disposable pattern and of the ceramic shell are transferred, so to speak, to the final casting. It is also apparent that the pattern material and any residue therefrom will affect the surface characteristics and metallurgical characteristics of a casting. Similarly, it is clear that variations in expansion and contraction of compositions from which disposable patterns are formed will result in shells or casts of varying dimensions, which will then produce inconsistent castings. It is for such reasons that the properties of pattern materials are critical to the investment caster.

Many thermoplastic pattern materials have been used and have been suggested for use in the past. As the name lost wax process implies, true waxes, such as natural waxes, as beeswax and the like, were originally used as thermoplastic pattern materials. As other pattern materials were sought to improve the properties of disposable patterns, other natural thermoplastic materials, such as gum damar, gum rosin, esparto, waxes, and the like, mineral waxes, such as those extracted from soft coal, and the like, and petroleum waxes were adopted for use. Subsequently, modified waxes, such as microcrystalline Waxes, were developed for use and used in lost wax processes. More recently, synthetic thermoplastic materials, such as polystyrene, have been used as pattern materials or as thermoplastic pattern forming composition modifiers as a result of the continuing efforts of researchers to improve upon and develop new thermoplastic materials. Those efforts have also resulted in the use by some investment casters of materials other than thermoplastic pattern materials, such as mixtures of metallic salts, mercury, among others.

Efforts have also been made to increase the dimensional accuracy and stability of thermoplastic pattern forming compositions by the addition of solid filler materials. Polystyrene powder and urea powder have been so used, and have been added in minor quantities to thermoplastic pattern-forming compositions. Organic acids, such as fumaric acid, adipic acid and isophthalic acid, have also sometimes been used as solid fillers, usually in amounts of up to about 40% by volume of the thermoplastic pattern forming composition, and in a particle size generally in the range of about 175 to 250 mesh.

In accordance with my invention, I have discovered that a thermoplastic pattern composition comprising an organic thermoplastic pattern material, and from about 5% to about and preferably from about 20% to about 70% of finely divided decachlorobiphenyl by weight of the thermoplastic pattern forming composition produces disposable thermoplastic patterns for use in investment casting processes which have superior di- -mensional reproduceability and highly accurate surface finishes, in addition to providing disposable patterns which have excellent characteristics in other important regards as well. Decachlorobiphenyl in this amount in pattern forming compositions of waxes, such: as true waxes, and resins used in lieu of waxes, produces a thermoplastic disposable pattern which has a number of advantages, and which has improved characteristics, as compared to those disposable thermoplastic patterns currently in vogue. The terms organic thermoplastic material or thermoplastic material are sometimes used in this application to include waxes and resins frequently referred to as waxes, such as polystyrene, and other presently useful or potentially useful waxes and natural and synthetic thermoplastic resins, such as rosin and polystyrene, which are usable as the thermoplastic portion of a thermoplastic pattern forming composition incorporating decachlorobiphenyl.

When disposable patterns are formed of organic thermoplastic materials and finely divided decachlorobiphenyi, upon cooling, they shrink and contract considerably less than the thermoplastic material itself would shrink. Other improved properties result from the use of decachlorobiphenyl in thermoplastic pattern forming compositions. For example, because decachlorobiphenyl is dimensionally and physically stable up to 305 C., a temperature considerably higher than the melting point of thermoplastic materials currently used to make disposable patterns, higher drying temperatures can be used during the application of ceramic coats to the thermoplastic patterns in the ceramic shell lost wax process. That speeds quite considerably the shell making portion of such investment casting processes. Additionally, because decachlorobiphenyi melts at 305 C. it far exceeds the melting point of the thermoplastic material into which it is incorporated. As such, the decachlorobiphenyl portion of the composition remains stable up to about 305 C., limiting the thermal expansion of the overall pattern, hence minimizing distortion of the pattern up to about 305 C. Further, decachlorobiphenyl is neither acid nor alkaline in its solid or liquid states, hence will not attack alkaline or acid sensitive ceramic materials.

Other advantages derived from the practice of this invention and of disposable patterns made in accordance with this invention will become apparent from the following description and examples.

The composition of this invention comprises a thermoplastic pattern material and decachlorobiphenyl in an amount of from 5% to about 70% by weight of the total thermoplastic pattern forming composition. For high quality castings, the particle should not exceed 100 mesh. Decachlorobiphenyl does not appreciably expand or shrink in a range from ambient room temperature to a temperature of 305 C. 1n pattern forming compositions decachlorobiphenyl is inert, hence it is not subject to shrinking upon cooling as are the lower melting thermoplastic portions of the thermoplastic pattern forming compositions.

Molecular weight 499 Chlorine content 7 l .77: Specific gravity c. 1.95 Melting point 305.0 to 305.5C. Boiling point at 760 mm 450 to 460C. Dielectric constant c. 3 Thermal expansion coefficient per C. 52 to 53 X 10" Compositions according to this invention have been formed into solid disposable patterns and have been tested.

EXAMPLE 1 A thermoplastic pattern material was made by melting and mixing ingredients as follows until they were homogeneously dispersed.

Ingredient Parts by Weight Terpene Polymer (115C. mp.) 55

Synthetic paraffinic mineral wax (200C. m.p.) 5 Paraffin 138 to 140F. mp) Natural carnauba wax l0 Microcrystalline wax (175-180C. mp.) 10 100 EXAMPLE 2 A mixture of 60 percent by weight of finely divided decachlorobiphenyl of between about and about microns in size and 40 percent by weight of the mixture of Example 1 was formulated. The decachlorobiphenyl was blended into the homogeneous melt of Example 1, but retained its particulate identity. The result was a two phase system with the decachlorobiphenyl particles uniformly distributed throughout. The ball and ring softening points of solid body test patterns cast from the compositions of Examples 1 and 2 were quite close. However, the volumetric expansions of the test patterns were markedly different, and as will be apparent, the volumetric expansion characteristics of the test pattern of Example 2 was very markedly improved.

Volumetric Expansion The accuracy of that test method is considered to be about i0.2%. The test results demonstrate a very substantial reduction in thermal expansion, a highly desirable property for disposable thermoplastic patterns.

It was also determined that the pattern of Example 2 was much less likely to break a ceramic cast when the pattern was eliminated. Additionally, the disposable pattern of Example 2 produced less residual ash and less erosion and wetting of the ceramic surface, thereby ultimately producing a casting having a more accurate surface finish. That is especially important in pattern removal processes where temperatures of as much as 1,600 to 2,000 F. are reached, thereby to eliminate wax and carbon.

Other compositions embodying the principles of this invention have been made. Disposable thermoplastic patterns formed from those compositions have demonstrated a reduction in ash content and a reduction in agitation caused by boiling and burning against the ceramic mold during de-waxing, and have also demonstrated improved reproduceability and surface finish characteristics.

Thermoplastic pattern forming compositions were blended, as indicated in Examples 1 and 2, of the following ingredients.

EXAMPLE 3 lngredient Parts by Weight Triple pressed stearic acid 40 Cumar resin MH so The ash content of Example 3 was about 0.013% of the original volume.

The ash content of Example 4 was found to have been reduced to about 0.007% of the original volume.

EXAMPLE 5 Ingredient Parts by Weight Mixture of Example 3 30 Powdered decachlorobiphenyl (30-40 micron size) 70 The ash content of Example 5 was found to have been reduced to about 0,004% of the original volume.

In Examples 4 and 5, the surface finishes of castings cast from a disposable pattern of the compositions indicated were found to be substantially improved as compared to the surface finish of castings cast from a disposable pattern made from the composition of Example 3.

As far as I am aware, efforts to use prior art fillers, such as those described above, with higher temperature waxes to form disposable patterns have failed, largely because such prior art fillers themselves decompose, or otherwise fail to function properly at elevated temperatures. When high temperature thermoplastic pattern materials, such as waxes, can be used to form patterns, they can speed the ceramic mold or cast forming step of an investment casting process because higher temperatures can be used to dry the ceramic shell or cast, thus decreasing the drying time. Higher melting thermoplastic materials also frequently exhibit greater dimensional stability, both at higher and lower temperatures, than do those lower temperature disposable pattern materials now in use, making the use of high temperature thermoplastic materials desirable. When decachlorobiphenyl is used, higher temperature waxes and other thermoplastic materials may also be used when desired.

EXAMPLE 6 A high temperature thermoplastic pattern material was compounded, as stated above, from the following Ingredient Parts by Weight Acrawax C (MO-143C. m.p.

This produced a twophase thermoplastic pattern composition which was quite stable and which did not melt below about 140C.

The high temperature characteristics of decachlorobiphenyl suggest other thermoplastic waxes" which might now be used as thermoplastic pattern materials, or which may be more useful than heretofore thought. For example, polystyrene resins tend to expand upon heating to an extent frequently unacceptable for the production of high quality investment castings. By using a substantial fraction of finely divided decachlorobiphenyl, a significant enough reduction in the expansion of polystyrene pattern material may be realized to make polystyrene powders useful where they were not before. For example, a thermoplastic pattern forming composition comprising 50 parts by weight of a methyl styrene copolymer (such as Resin EXAMPLE 7 Ingredient Parts by Weight Crystalline Polyolefin resin, 270F. mp. 50

Powdered decachlorobiphenyl to micron size) 50 100 EXAMPLE 8 Ingredient Parts by Weight Ethylene isobutyl acrylate copolymer,

250i25F. mp.

Powdered decachlorobiphenyl (30 to 40 micron size) 40 I00 EXAMPLE 9 Ingredient Parts by Weight Ethylene ethyl acrylate copolymer. IF. mp. 75

Powdered decachlorobiphenyl (30 to 40 micron size) 25 I00 EXAMPLE l0 Ingredient Parts by Weight Microcrystalline wax, l-l95F. m.p.

Powdered decachlorobiphenyl (30 to 40 micron size) Other thermoplastic pattern materials which appear to have potential and which produce improved thermoplastic patterns because of the presence of decachlorobiphenyl are the following:

EXAMPLE l2 ingredient Parts by Weight Paraffin (l38l40F. m.p.) 4 Beeswax IO Gum damar 7 Carnauba wax 2O Cumar resin (.l40C. mp.) 49 Polyethylene l Powdered decachlorobiphenyl (30 to 40 micron size) 40 the foregoing. It is my intention that those equivalent compositions shall also be considered as being within the scope of the appended claims.

I claim:

1. In a two-phase thermoplastic pattern forming composition comprising a thermoplastic pattern material and a finely divided solid filler material, the improvement consisting essentially of utilizing, as said filler material, decachlorobiphenyl having a chlorine content of about 71.5% and a melting point higher than that of the thermoplastic pattern material and present in an amount from about 5% to about by volume of the pattern forming composition.

2. A thermoplastic pattern forming composition in accordance with claim 1 in which said pattern material is a true wax.

3. A thermoplastic pattern forming composition in accordance with claim 1 in which said thermoplastic pattern material is a polystyrene resin.

4. A thermoplastic pattern forming composition in accordance with claim 1 in which said decachlorobiphenyl is present in an amount of from about 20% to 70% by weight. 

1. In a two-phase thermoplastic pattern forming composition comprising a thermoplastic pattern material and a finely divided solid filler material, the improvement consisting essentially of utilizing, as said filler material, decachlorobiphenyl having a chlorine content of about 71.5% and a melting point higher than that of the thermoplastic pattern material and present in an amount from about 5% to about 70% by volume of the pattern forming composition.
 2. A thermoplastic pattern forming composition in accordance with claim 1 in which said pattern material is a true wax.
 3. A thermoplastic pattern forming composition in accordance with claim 1 in which said thermoplastic pattern material is a polystyrene resin. 